These machines are driven by a motor and they have a high flow rate, greater than 50 m.sup.3 /h, and they rotate at speeds higher than 10,000 revolutions per minute (rpm). At atmospheric pressure they are capable of maintaining suction down to a pressure of 10.sup.-2 mbar. At a suction pressure in the range 200 mbar to 300 mbar, they absorb large amounts of power which is dissipated entirely as heat. Since the rotor and the stator are very different in thermal mass and in thermal dissipation, it is necessary to place considerable clearance between the rotors and the stator in order to avoid seizing. This gives rise to a poor quality flowrate/pressure characteristic.
An object of the present invention is to improve the flow rate-pressure characteristic by taking measures that make it possible to reduce the clearance between the rotors and the stator.